This disclosure relates to a microwave cavity resonator.
A microwave cavity resonator of this kind comprises a cavity housing forming a cavity, the cavity housing comprising a first housing wall and a second housing wall opposite the first housing wall. A resonator element is arranged in the cavity and extends longitudinally along a longitudinal axis. The resonator element comprises, when viewed along the longitudinal axis, a first end connected to the first housing wall and a second end opposite the first end, the second end being arranged at a distance from the second housing wall.
A microwave cavity resonator of this kind may for example be used in a microwave filter, for example a band pass filter or a band stop filter, in a multiplexer or in another radiofrequency (RF) device.
A microwave filter including multiple cavity resonators is for example known from U.S. Pat. No. 6,735,766. A resonator element placed within a cavity of a cavity resonator is herein attached with its first end to a bottom wall of a cavity housing and with a second end is arranged at a distance from a housing cover opposite the bottom wall. The second end of the resonator element hence represents an open end which is not connected to the housing cover.
Within a cavity resonator of this kind the resonator element comprises an electrical length of a quarter wavelength at the resonant frequency of the cavity resonator. This poses a limit for the dimensions of such cavity resonators, which may be in contrast to a desire for a miniaturization and for low production costs.
It has been proposed to place a capacitor element at the second, open end of the resonator element in order to increase the capacitance in-between the second, open end of the resonator element and the surrounding cavity housing. This allows to shorten the resonator element.
In EP 1 118 134 B1 a cavity resonator is described in which discs are placed in the vicinity of the second, open end of the resonator element, the discs electrically interacting with plates on the cavity housing in order to increase the capacitance in-between the second, open end of the resonator element and the cavity housing.
There is a desire to increase the capacitance between the second, open end of the resonator element and the surrounding cavity housing further. Herein, in known solutions, it is necessary to arrange the second end of the resonator element with respect to the surrounding housing such that a relatively small gap in-between the second end of the resonator element and one or multiple housing walls of the housing is obtained. Such arrangements are sensitive to tolerances and sometimes are mechanically and electrically unstable over the operational range of temperatures.
From U.S. Pat. No. 3,448,412 a miniaturized tunable resonator is known in which a movable assembly, together with a cylindrical member, forms a structure similar to a folded coaxial line and hence a resonator within a cavity.